The burden of the demands for improved performance of computers falls on the lithographic process used to fabricate the integrated circuit chips. Lithography involves irradiating a mask and focusing the pattern of this mask through an optical microlithography system onto a wafer coated with a photoresist. The pattern on the mask is thereby transferred onto the wafer. Decreasing the line-widths of the features on a given wafer brings about advances in performance. The enhanced resolution required to achieve finer line-widths is enabled by decreasing the wavelength of the illumination source. The energies used in lithographic patterning are moving deeper into the UV region. Optical components capable of reliable performance at these short optical microlithography wavelength are required. Few materials are known that have a high transmittance at 193 nm and 157 nm and do not deteriorate under intense laser exposure. Fluoride crystals such as calcium fluoride and barium fluoride are potential materials with high transmittance at wavelengths less than 200 nm. Projection optical photolithography systems that utilize the vacuum ultraviolet wavelengths of light at and below 193 nm provide desirable benefits in terms of achieving smaller feature dimensions. Microlithography systems that utilize vacuum ultraviolet wavelengths in the 157 nm wavelength region have the potential of improving integrated circuits and their manufacture. The commercial use and adoption of 193 nm and below vacuum ultraviolet wavelengths such as 157 nm has been hindered by the transmission nature of such deep ultraviolet wavelengths in the 157 nm region through optical materials. Such slow progression by the semiconductor industry of the use of VUV light below 175 nm such as the 157 nm region light has been also due to the lack of economically manufacturable blanks from optically transmissive materials and difficulties in manufacturing blanks which can be identified as high quality and qualified for their intended microlithography use. For the benefit of deep ultraviolet photolithography in the VUV 157 nm region such as the emission spectrum of the fluorine excimer laser to be utilized in the manufacturing of integrated circuits there is a need for FLUORIDE CRYSTALLINE OPTICAL LITHOGRAPHY LENS ELEMENT BLANKS that have beneficial optical and highly qualified crystalline properties including good transmission below 200 nm and at 193 nm and 157 nm and that can be manufactured and qualified economically. The present invention overcomes problems in the prior art and provides a means for economically providing high quality identifiable FLUORIDE CRYSTALLINE OPTICAL LITHOGRAPHY LENS ELEMENT BLANKS that can be used to improve the manufacturing of integrated circuits with vacuum ultraviolet wavelengths.
The invention comprises a high quality identifiable fluoride crystalline optical lithography lens element blank. The fluoride crystalline optical element blank includes crystalline subgrains which have crystalline subgrain structures. The fluoride crystalline optical element blank includes at least a first subgrain structure and a second subgrain structure. The second subgrain structure is adjacent to and abuts the first subgrain structure at a first defect boundary formed by dislocation defects. The first defect boundary has an adjacent first subgrain-second subgrain boundary angle. The first subgrain-second subgrain boundary angle is less than two minutes and the blank has an impurity level less than 1 ppm Pb by weight, less than 0.5 ppm Ce by weight, less than 2 ppm Na by weight and less than 2 ppm K by weight. The blank has a 157 nm internal absorption coefficient less than 0.0022/cm (base 10 absorption coefficient) and a 193 nm internal absorption coefficient less than 0.000431/cm (base 10 absorption coefficient), with an optical homogeneity less than 2 ppm and an average birefringence less than 2 nm/cm RMS with a maximum birefringence less than 5 nm/cm.
In a preferred embodiment the invention includes a method of making a fluoride crystalline optical lithography lens element blank. The method of making includes forming a fluoride crystalline melt, crystallizing the melt into a fluoride crystalline member preferably with a large dimensionxe2x89xa7200 mm, and annealing the fluoride crystalline member. The method further includes qualifying the annealed fluoride crystalline member to provide a fluoride crystalline optical lithography lens element blank with a 157 nm internal absorption coefficient less than 0.0022/cm and a 193 nm internal absorption coefficient less than 0.00043/cm, a 205 nm lead absorption less than 0.23 cmxe2x88x921, an average birefringence less than 2 nm/cm with a maximum birefringence less than 5 nm/cm, and an optical homogeneity less than 2 ppm with a maximum surface subgrain disorientation boundary anglexe2x89xa62 minutes.